1. Field of the Invention
The present invention relates to the field of oil-well equipment used in an intermediary region between a floating production unit at the ocean surface and a wellhead of an oil reservoir, or well, on the ocean floor. The equipment is designed to support rigid and flexible production tubes, or pipes, used for the transport of reservoir fluids from the oil well and/or fluids used in support systems for the oil reservoir. In addition, a method of installing the equipment at its designed operating depth is also disclosed.
2. State of the Art
Offshore exploration of hydrocarbons in deep or ultra-deep waters has always presented a technological challenge to companies involved in such an undertaking and represents a significant boundary, or limit, to applicable scientific knowledge. This challenge, when overcome, will take us to the forefront of this technology.
Given the large depths associated with deep-water exploration and, consequently, the exposure to a harsh environment, interconnection between the wellhead on the ocean floor and a flow system for extracting the oil production is a great technological challenge. In addition, the installation and support of ascending production tubes, also known as xe2x80x9crisersxe2x80x9d in a configuration known as suspended catenaries, adds to the technological challenge.
In the environment of ultra-deep water, it has been known that hybrid concepts, if not considered the only alternative due to limited field experience, should be evaluated with much care. This need of evaluation exists due to the use of flexible risers as an interconnection between the wellhead on the ocean floor and the production unit. Flexible risers in suspended-catenary configuration, i.e., a direct connection between the wellhead and the floating production unit for depths greater than 1000 meters, present dynamic problems caused by movement of the floating production unit itself. These movements may provoke a compression and/or excessive curvature at the location where the catenary, formed by the riser, contacts the ocean floor or creates an additional load at the top of the riser due to angular motion and/or horizontal displacement that may provoke rupture in the connections at the surface. In the case of the flowing unit being a Floating Production Storage and Offloading (FPSO) ship, the above mentioned problems exist in a much more critical scale.
Another type of riser which is used is a substantially rigid riser, known as Steel Catenary Riser (SCR). The use of SCRs, connected directly to the floating production units, has limitations, as far as their configuration when used in ships converted to production (FPSO), because the ship movements provoked by waves are more accentuated compared to semi-submersed platforms. SCRs may have desirable characteristics associated with the capacity to support, along their extremities, higher tension as compared to flexible risers; however, the flexible risers have longer life in terms of resisting fatigue even taking into account their interaction with ocean waves and currents.
It has been, therefore, proposed to combine the two types of risers in an assembly in order to take advantage of the best characteristics of each type: namely the resistance to tension and the higher economic viability associated with SCRs; and a significantly higher resistance to fatigue associated with flexible risers.
In an assembly, an intermediary system would be necessary, having sufficient floatation to support the weight of the rigid production tubes, while, at the same time, exhibiting only small displacements in response to the horizontal loading of SCRs and the environmental working loads. The system, or assembly, would also need to be submersed sufficiently to protect it from the effects of waves at the ocean surface, be capable to connect the SCR to the flexible riser, and be reasonably easy to install. With these necessary prerequisites, the concept of a subsurface buoy, or float, developed naturally, permitting a considerable reduction in the weight of the production tubes on the floating unit, improving undersea arrangement, and thus making possible the use of a hybrid system of risers.
Hybrid systems of risers, based in a subsurface buoy, have recently been recognized as an alternative to the limitations found in petroleum production activities in deep-sea water.
There exist in the art, and more specifically in the field of introduction and connection of production tubes, concepts of intermediary support systems to promote the connection between a vertical riser and flexible tubes or even concepts to reduce the loads on equipment and tubes.
Examples of these systems may be found in the documents BR/PI 9202379-A, belonging to Bechtel Limited, in which a system to deploy, or unfold, cable used at an intermediate floating level and the associated flexible rising tubes are disclosed. U.S. Pat. No. 4,423,984, assigned to Mobil Oil Corporation, discloses a way to interconnect the flexible tubes originating at a surface unit and a rigid vertical riser with a buoy connected at the upper extremity of the vertical riser. U.S. Pat. No. 5,007,482, assigned to British Petroleum Company, similarly discloses another concept to make the connection between the wellhead and a floating unit using a buoy as an intermediary support for the flexible tubes.
Although the inventions just summarized may appear as viable solutions, they may become problematic when considering their economic viability, installation difficulties, and behavior when exposed to the harsh environmental conditions such as, for example, the effect of the waves.
The present invention relates to equipment designed to support and interconnect rigid and flexible tubes used in the production and transport of oil from an underwater well and/or fluids used in support systems for the reservoir. Such equipment is usually known as a subsurface buoy. A system to tie and to dynamically stabilize the buoy is also in the scope of the present invention. Additionally, a method is also disclosed to install the subsurface buoy at its location of operation.
The subsurface buoy, one of the objects of the present invention, comprises four interconnecting cylindrical units forming a single unit, each one of these cylindrical units having in its interior a plurality of draining compartments for the purpose of ballasting. Such a floating body resembles a quadrilateral frame defining a whole therethrough, having a plurality of fixed connections for interconnecting the rigid and flexible tubes thereto. A tying and dynamically stabilizing system, rigidly connected at each of the vertices of the subsurface buoy, is another object of the present invention. The tying and dynamically stabilizing system is designed to control the float position and the traction and the tension in the anchoring tendons, providing stabilization for the entire unit against large-amplitude rotation or changes in angular position even after the rigid and flexible tubes are couple or connected to the floating body, or buoy. Hereinafter, chained sections formed by links will be referred to as xe2x80x9cchainsxe2x80x9d and steel or polyester cables will be designated as xe2x80x9ctendons.xe2x80x9d
Another object of the present invention is a method of installation of the subsurface buoy at its operating location with the fluid transport tubes connected thereto and the use of a system to tie and dynamically stabilize the subsurface float.